Flame spray apparatus has been employed for melting powder particles in effecting the build-up of molten material on a substrate downstream of the discharge and of the flame-spray duct. Additionally, such flame-spray apparatus modified or unmodified has been employed in accelerating small abrasion particles in the flame-spray at high velocity, sonic or supersonic, with the particles ejected at high speed from the discharge and of the flame-spray apparatus to impact a workpiece positioned in the path of particle flow. Internal burner type supersonic flame-spray apparatus of this type is exemplified by my U.S. Pat. No. 2,990,653 issued July 4, 1961; 4,370,538 issued Jan. 25, 1983; and U.S. Pat. No. 4,416,421 issued Nov. 22, 1983.
By the utilization of an expansion nozzle of extended length, products of combustion from an internal burner to which an oxy-fuel mixture is continuously fed and combusted, with combustion effected under substantially high pressure and with the products of combustion directed through a flow expansion nozzle, the creation of a flame-spray stream of supersonic velocity is achieved. Aspects of these patents include the mode of introduction of solid material in either rod form or particle form into the flame-spray and an attempt to insure a concentrated and highly focused core of flame-spray material for material spray coating downstream of the nozzle at supersonic velocity, or alternatively, the spraying of abrasive solid particles for sandblasting of a workpiece positioned in front of the nozzle exit port.
My published European patent application No. 0 163 776, published Nov. 12, 1985, is directed to such supersonic flame-spray apparatus utilizing an internal burner which continuously feeds oxy-fuel products of combustion through an extended length nozzle of diminished throat area and a supersonic expansion duct 15 upstream of an extended length nozzle passage 16, 36. The expansion of the hot products of combustion from the throat 14, 35 to the end of the expansion duct 15 provides a gas velocity well in excess of supersonic. In this supersonic flame apparatus, solid material in rod or particle form is introduced axially or radially into the supersonic gas flow at the end of the expansion duct 15 and allows the extended length nozzle passage to have a relatively large diameter tending to prevent solid material introduced to the flow to melt, reach the nozzle passage wall and adhere there to or, where abrasive particles form that solid material, abrade the nozzle wall. Additionally, the material injection can be achieved at low pressure, since the hot products of combustion are at significantly reduced diameter but supersonic velocity at the discharge end of the expansion duct 15.
While the present invention is directed to a duct stabilized cold gas flame-spray method and apparatus, certain aspects of internal burner supersonic flame-spray apparatus of the type described above, include common problems and employ structural elements necessary to cold gas flame-spray apparatus.
The lessons learned from the developments in the internal burner supersonic flame-spray apparatus form a background and have led to an understanding and appreciation of problems common to both hot gas supersonic and cold gas duct-stabilized flame-spray method and apparatus and have led to the improvement within the duct-stabilized cold gas flame-spray method and apparatus forming the invention.
In that regard, attempts to use satisfactory duct-stabilized flame-spray apparatus have been unsuccessful to date without the use of complex geometries. The main difficulty has been found to lie in the overheating of powder particles when using uneven gas flow regimes. Recirculation of the particles within the flame occur. The smaller particles may melt causing the build-up of adhering masses of material against the nozzle and flame passage walls.
It is therefore a primary object of the present invention to provide an improved duct stabilized cold gas flame-spray method and apparatus for flame spraying molten material on a substrate or flame spraying abrasion particles against the workpiece for surface blasting of the same, and which achieves even gas flow regimes at the point of introduction of solid material in power particle form into the flame-spray and during flow downstream thereof through the flame-spray duct.